Treatment of equine onchocerciasis with ivermectin paste

A single oral dose of ivermectin paste was administered to 12 horses with dermatitis and clinical signs typical of onchocerciasis. Two of the horses also had lesions of Queensland itch. Microfilarias of Onchocerca cervicalis were identified in fresh, macerated, skin biopsies from the neck, brisket or umbilical regions of all horses and microfilarias of O. gutturosa from the neck of 2. Eight of the horses developed skin reactions 4 to 24 h after the administration of the ivermectin, notably weals over the neck, shoulders and flanks and pitting oedema of the ventral midline and intermandibular space. Regression of the onchocerciasis lesions was evident within 7 days of treatment and affected skin had returned to normal within 3 months. The lesions of Queensland itch were not affected by the ivermectin treatment. Microfilarias were present in biopsies of skin, particularly in the superficial dermis, before treatment, but were absent from all skin biopsies taken one week after treatment. In some horses transient skin sensitivity reactions developed. Microfilarias began to reappear in the biopsies of some of the horses 2 months after treatment. It is concluded that the oral paste formulation of ivermectin, although not effective against adult onchocerca, is useful for the therapeutic control of microfilarias in the skin lesions of equine onchocerciasis.